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arXiv · 2608.00174

On Bimodality in the Eccentricity Distribution of Galactic Double Neutron Stars

Aldana Grichener·Paul Disberg·Ryosuke Hirai·Ilya Mandel
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Abstract

The detection of Galactic double neutron stars (DNSs) through pulsar timing offers a unique opportunity to probe massive stellar and binary evolution. The observed DNS population exhibits an apparently bimodal eccentricity distribution, with an absence of systems at measured intermediate eccentricities, $0.4 \lesssim e_{\rm m} \lesssim 0.58$, whose origin remains unclear. We propose that this possible gap can arise naturally if the relationship between the progenitor masses and neutron star (NS) masses is non-monotonic, provided that the second-born NS receives a sufficiently small natal kick. We illustrate this scenario using the population synthesis code COMPAS, and find that our DNS population model can reproduce the observed orbital period-eccentricity distribution relatively well, including the apparent bimodality. Although a larger observed sample is required to draw more robust conclusions, our results suggest that this model provides a natural pathway for explaining current observations of Galactic DNSs through isolated binary evolution.

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Aldana Grichener, Paul Disberg, Ryosuke Hirai, Ilya Mandel. 2026-07-31. On Bimodality in the Eccentricity Distribution of Galactic Double Neutron Stars. https://arxiv.org/abs/2608.00174

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